The present invention relates to an optical compensation sheet and its fabrication method as well as a liquid crystal display device using such a sheet. In particular, this invention is concerned with an optical compensation sheet useful for improving displayed contrast and the visual angle property of displayed color and a liquid crystal display device making use of such a sheet.
For display devices for word processors or personal computers, esp., miniature ones, liquid crystal display devices having the major advantages of low weight, small size and low power consumption are now widely used in place of CRTs. Most of currently available liquid crystal display devices (hereinafter LCDs for short) make use of twisted nematic liquid crystals. Devices made up of such liquid crystals work generally on two modes, a birefringent mode and an optical rotary mode.
In the case of the LCD working on the birefringent mode, sole use of a simple matrix form of electrode structure enables a massive display to be obtained by time-sharing addressing even in the absence of active elements such as thin-film transistors or diodes. This is because the twist angle of the molecular arrangement of liquid crystal is 90.degree. or more, making the electro-optic property sharp. However, this LCD has a grave disadvantage of having a response speed as slow as a few hundred milliseconds, making gradation displays difficult, and so is still lower in display performance than a TFT or MIM display device using active elements.
The TFT or MIM type display work on the optical rotary mode wherein the liquid crystal molecules are twisted 90.degree. (TN liquid crystal display elements). This display mode appears to be the most promising over other LCDs, because the response speed is as fast as scores of milliseconds and black-on-white displays can be easily achieved with high contrast. However, this display device is again still lower in display performance than CRTs, because of its own visual angle property that depending on the viewing direction there are variations in the displayed color and contrast.
As set forth in JP-A-4-229828, 4-258923, etc., it proposed to locate a phase difference film between a pair of polarizing plates and a TN liquid crystal cell, thereby making the angle of field large.
The phase difference film proposed in these specifications is substantially in phase with the surface of the liquid crystal cell in the vertical direction, and so cannot have any optical action on the liquid crystal cell, when it is just in front thereof. If tilted, however, the phase difference film then gives rise to a phase difference with respect to the liquid crystal cell, thereby making up for a phase difference occurring on the liquid crystal cell. Even with this method, however, the angle of field of the LCD is still less than satisfactory and, in this regard, there is left much to be desired. Especially when used on cars or as an alternative to CRTs, conventional LCDs are of no use in terms of the angle of field. As disclosed in JP-A-4-366808, 4-366809 and 4-366810, it has also been proposed to use a chiral nematic liquid crystal with an inclined optic axis as a phase difference film, thereby improving the angle of field. However, this leads to a costly, very heavy bilayer liquid crystal system.
A primary object of the present invention is to make some considerable improvement in the narrow angle of field resulting from the fact that the polarized state of light propagating through a liquid crystal cell varies depending on the incident direction, thereby achieving a wide angle-of-field property alternative to that of a CRT with low weight and at low cost.